Ladder Problem

Hello, I created a script which checks whether you're inside the ladder collider and if you are and hold C it moves you up. The problem is that when I hold C it goes me up but then I fall down all the time...

What's the problem?

Here's the code:

 var inside : boolean = false;
 private var FPSInput : FPSInputController;
 
 function Start() {
     FPSInput = GetComponent(FPSInputController);
 }
 
 function OnTriggerEnter(col : Collider) {
     if(col.gameObject.tag == "Ladder") {
         inside = true;
     }
 }
 
 function OnTriggerExit(col : Collider) {
     if(col.gameObject.tag == "Ladder") {
         inside = false;;
     }
 }
 
 function Update() {
     if(Input.GetKey(KeyCode.C) && inside) {
         this.transform.position.y = this.transform.position.y + (5*Time.deltaTime);
         FPSInput.enabled = false;
     }
     if(Input.GetKeyUp(KeyCode.C) && inside) {
         FPSInput.enabled = true;
     }
 }